1. Field of the Invention
The present invention relates to an apparatus for measuring radial and axial displacements of a rim of a disk wheel.
2. Description of the Prior Art
As illustrated in FIG. 12, a disk wheel 1 includes a rim 2 and a disk 3. Rim 2 includes a bead seat portion 4 which contacts a tire bead in a radial direction and a flange portion 5 which contacts the tire bead in an axial direction. As illustrated in FIG. 13, disk 3 includes a hub hole 6 at a center of the disk and a plurality of nut holes 8 around hub hole 6. A nut seat 7 is formed at a surface of a portion of the disk defining an outboard end of each nut hole 8 and is tapered. Wheel 1 is secured to an axle shaft 90 of a vehicle by causing a hub bolt 91 fixed to axle shaft 90 to penetrate nut hole 8 and then causing hub nut 92 to screw-engage hub bolt 91. Nut seat 7 contacts a semispherical portion of a head of hub nut 92, whereby wheel 1 is positioned relative to axle shaft 90. In this structure, a clearance cl exists between a surface of hub hole 6 and a outside surface of a portion of axle shaft 90 penetrating hub hole 6, and a clearance c2 exists between an outer surface of hub bolt 91 and a surface of nut hole 8.
In FIG. 12, a radial displacement of an outer surface of bead seat 4 from a true circle having a center at an axle shaft axis and an axial displacement of an inboard surface of flange 5 from a plane perpendicular to the axle shaft axis are required to be less than respective allowable limits to obtain a good drive feeling. Since a rim is manufactured through the steps of rounding a flat plate, welding both ends of the rounded plate to form a cylindrical element, flaring opposite end portions of the cylindrical element, forming the flared cylindrical element into a rim element having a rough rim contour, and shaping the rim element to a rim element having an exact rim contour using an expander and/or shrinker, various kinds of distortions and strains remain in the exactly shaped rim. Therefore, measurement of radial and axial displacements of the rim and a judgment regarding whether or not the measured displacements are within allowable limits should be carried out at a final stage of a wheel manufacturing process.
According to a prior art apparatus for measuring wheel rim displacements, as illustrated in FIG. 14, a wheel 1 to be measured is mounted on a turn table 101. Then, a spindle 104 placed above turn table 101 is lowered by a cylinder 105 and pushes a collet chuck 103. When collet chuck 103 slidably engages a taper cone 102 fixed to turn table 101, collet chuck 103 is radially expanded and is pushed against a hub hole defining surface of wheel 1 whereby wheel 1 is secured to turn table 101. A radial displacement sensor (not shown) contacts a radially outer surface of bead seat 4 of wheel 1, and an axial displacement sensor (not shown) contacts an axially inboard surface of flange 5 of wheel 1. Then, wheel 1 is rotated by rotating turn table 101 and radial and axial displacements of the wheel rim are measured. In this measurement, displacements of the rim wall from a true circle having a center at a hub hole center are measured, because the wheel to be measured is secured to the turn table on the basis of the hub hole center. In this instance, if the hub hole center is misaligned from the turn table axis, the measurement results will contain errors caused by misalignment.
The above-described prior art measurement includes the following problems:
First, in mounting a wheel on a vehicle, since wheel 1 is positioned on the basis of nut seat 7 and secured to axle shaft 90, the center of hub hole 6 does not always coincide with the axis of axle shaft 90, though the center of the nut seat pitch circle coincides with the axis of axle shaft 90. More particularly, the center of hub hole 60 can be misaligned from the axis of axle shaft 90 within the limit of a clearance cl of FIG. 13. Therefore, measurement of displacements of a wheel rim from a true circle should be performed not on the basis of the center of the hub hole, but on the basis of the center of the nut seat pitch circle. For example, when a wheel, whose displacements measured on the basis of the hub hole center are within an allowable limit, is secured to an axle shaft, displacements measured on the basis of the nut seat pitch circle center may exceed the allowable limit because the displacements are added to the distance between the hub hole center and the nut seat pitch circle center.
Second, in order to develop an apparatus for measuring wheel rim displacements on the basis of a nut seat pitch circle center, the space above the turn table is preferably used as a space for placing a device for defining the nut seat pitch circle and the center of the circle. For this reason, it is not preferable to place the spindle 104 and the cylinder 105 for fixing the wheel in the space above turn table 101 as shown in FIG. 14. More particularly, the device for detachably securing wheel 1 to turn table 101 should be provided on the side of turn table 101 and such an apparatus is preferably developed.
Third, a detailed method for computing wheel rim displacements from a true circle having a center at a nut seat pitch circle center from various data which are measured on the basis of a hub hole center should be developed.